Audio signals are frequently transmitted in a digital format as a sequence of digital numbers as used in common consumer products such as compact disc players, mobile phones, and the like. Such products typically convert analog audio streams to digital audio data streams for transmission, e.g., over mobile telephone networks.
Two digital audio data streams are often combined to produce a combined input audio data signal. This process of combining multiple audio data streams is commonly called “mixing.” In order to mix by adding together two or more input digital audio data streams, the input audio data streams need to have the same sample rate, or the resulting audio will be distorted.
However, in many cases where two or more digital audio data streams are to be mixed, the incoming digital audio data streams do not have the same sample rate. The original sample rate of the audio may have been generated or recorded at different sample rates, such as, for example, 48,000 samples per second for high fidelity audio, and 8,000 samples per second for telephone quality audio.
Even if the original audio sources are generated at the same nominal sample rate, the crystal oscillators commonly used for clocking audio data can cause some error or drift. Typically, lower end crystal oscillators may drift by 1 part in 10,000 or 100 parts per million (PPM).